Circuit scheme for suppression of carrier waves



CIRCUIT SCHEME RoR SUPPRESSION oF CARRIER WAVES Sfr Filed July l, 1932 Maffo/ H. O. ROOSENSTEIN 0 F. mw 5 w /r/ff El-WM l |NvENToR HANS o. RooseNsmN BY #SZ ATroRNEY Patented Mar. 24, 1936 UNITED STATES PATENT OFFICE CIRCUIT SCHEME FOR SUPPRESSION F CARRIER WAVES tion of Germany Application July 1, 1932, Serial No. 620,314 In Germany July 2, 1931 4 Claims.

This invention relates to a system for the suppression of a carrier wave where it is desirable that only one or, at most, two side-band frequencies should be received. Various methods are well known in the art for suppressing either a side-band or a side-band and a carrier-wave. For this purpose, recourse is sometimes had to a quartz crystal mechanical filter stopper element of low damping. Also the use of other mechani- 0 cal oscillators, for instance, the kind predicated for its operation upon the phenomenon of magneto-striction would be feasible in this connection.

It has been discovered that for an oscillatory quartz crystal whose damping is so high that a side-band of the transmitted oscillation is effectively suppressed, there occurs also a considerable suppression of the other side-band. But if the damping of the quartz crystal be further diminished then also the undesired side-band will no longer be adequately weakened.

Now, according to the present invention a further improvement of this condition is feasible by the use o f a filter chain comprising a plurality of quartz crystals, the latter being used partly as series and partly as transversal units or else in combination with circuit schemes containing tubes or other circuit elements.

Figure l, for instance, shows an arrangement comprising two crystals one of which is arranged in series, while the other one is shunted.

Figure 2 shows an arrangement comprising two crystals in series in conjunction with amplier tubes 3 and 4.

Figure 3 shows diagrammatically an arrangement wherein the crystal lter chain affects a frequency band which has a width corresponding to the side-band to be stopped.

Figure 4 shows a piezo-electric device having mechanical damping means associated therewith.

Referring to Figure 2, there is shown fed to the terminals 5 and 6 the normal modulated radio frequency signal. Under certain circumstances it may be of advantage to suppress the carrier wave of this signal by the use of such means as are known in the prior art.

In circuit schemes of the kind as above outlined it is important that all of the quartz crystals used should have exactly the requisite natural waves. Inasmuch as this is feasible only with great difficulty by means of grinding it is advantageous to use the various approximately correctly ground crystals confined within thermostats, the temperatures of which must be so adjusted that the liminal frequencies of the filter chain will assume the correct value and that the cut-oif or boundary should be sharp at this frequency. For subsequent correction of such errors as may still be found the use of an adjustable thermostat is of advantage.

In order that proper operation of the arrangement hereinbefore described may be secured the constituent elements of the crystal filter chain should possess such high dampings that it will effectively stop the undesired side-band. This condition is attainable by th-e artificial adjustment of the damping of the quartz crystals. Most convenient for this purpose is the use of mechanical means, e. g., the enveloping of the quartz crystal inside a damping or deadening medium.

However, favorable results are attainable also with varying the damping of the quartz crystal by so choosing the frequency of the modulated signal which is fed to the lter chain that the above condition will be satisfied. Inasmuch as this frequency is generally not equal to the frequency which is to be transmitted, the signal appearing at the output end of the filter chain is adjusted to the proper frequency value by such means as are known in the earlier art for transforming the frequency.

Figure 3 shows such an arrangement diagrammatically. An input stage V furnishes the optimum modulated frequency fl suited for the quartz crystal filter chain SI. Suppose the modulator frequency is 1L. The filter chain Sl filters out of the mixture the frequency fl-l-n. Thereupon a radio frequency wave f2 is added in the modulator stage M following after chain SI, said radio frequency wave having a value such that fl+f2 (or l-f2) is the frequency which is to be transmitted. A filter chain S2 which may consist of standard inductance coils and condensers as usually employed insures the elimination of the undesired frequencies and allows only of the transmission of the side-band fl-l-JZ-l-n.

Figure 4 shows by way of example certain preferred structural details of a piezo-electric device having associated therewith means for damping the vibratory characteristics of the crystal. The crystal l is shown resting upon one electrode 1 and being suitably spaced from an electrode 8 which may be supported in any desired manner by a crystal holder cover 9. An insulating spacing member l5 is provided between the base 1 and the cover 9. The electrode 8 is preferably hollowed out in the center and has a centrally disposed orifice through which a threaded stud ll may be inserted. This stud has a plate lll mounted on the lower end thereof and an adjusting knob I2 at the upper end and between the plate I and the crystal I a layer of felt I4 or other suitable damping material may be placed. The pressure applied tothe crystal through the felt is adjusted by turning the knob I2 to a suitable position and then locking the stud in place by means of a nut I3.

From the foregoing description it will be clear that any desired degree of damping may be obtained, depending upon whether the carrier frequency or'one of the side bands is to be suppressed.

To suppress the carrier frequency -very little damping is required, the value of the latter depending on the constancy of this frequency; in choking a side band the damping must be about 1r times larger than the ratio width of sideband in kc frequency in kc The required selectivity depends on whether it is only desired to save transmitting power or whether one wishes to eliminate the band as completely as possible,` with a view to reducing disturbances or interference. In the rst case a selectivity cf 1:10 is required which can be accomplished, when speech is heterodyned, by means of two quartz crystals. In the second case a selectivity of 1:100 or more is needed, which requires at least a four-link chain of the described kind.

I claim:

1. In a signaling system, in combination, an electron discharge device having a cathode and control electrode, an input circuit comprising a source of modulated waves, a piezo-electric device serially connected between said control electrode and said source, another piezo-electric de- Vice connected in shunt between said control electrode and said cathode, and damping means for both of said crystals.

2. In a signaling system, in combination, an electron discharge device having a cathode and a control electrode, an input circuit comprising a source of modulated waves, a piezo-electric device serially connected between said control electrode and said source, and another piezo-electric device connected in shunt between said control electrode and said cathode, both of said piezoelectric crystals being enclosed in a controllable damping medium.

3. The combination with an electric wave lter including a piezo-electric crystal as a circuit element, said crystal having a very sharp selectivity characteristic, of means for controllably damping the vibration of said crystal whereby its sharpness may be reduced.

4. The combination with an electric wave lter including a piezo-electric crystal as a circuit element, said crystal having a very sharp selectivity characteristic, of mechanical means for controllably damping the vibration of said crystal whereby its sharpness may be reduced.

HANS O. ROOSENSTEIN. 

